Quieting Mass Extinction Worries

I don’t want to leave the subject of gamma ray bursts (GRBs) without considering findings that seem to reduce the potential threat from these events. And the revision of a significant GRB paper that I meant to discuss earlier gives me the chance to circle back around to it. The subject is intriguing because it bears on the spread of life in the cosmos. If gamma ray bursts — powerful flashes of energy emitted in narrow jets — are nearby, an evolving species might be destroyed before it could ever achieve sentience, much less technology.

Krzysztof Stanek (Ohio State University) and collaborators approach the GRB question assuming that long gamma ray bursts (two seconds or more in duration) result from the death of massive stars. They also note two further facts about the unusual events. GRBs are highly beamed, and the supernovae remnants they leave behind are deficient in both hydrogen and helium in their spectra. And then we add this: Compared to average galaxies, those hosting GRBs appear to be much less luminous.

Using these findings, Stanek studies five low redshift GRBs, so called ‘local’ bursts, all of which were followed by well documented supernovae. The word ‘local’ is, of course, by comparison only. As opposed to far more distant GRBs, the highest redshift in this sample corresponds to a look back time of about 2/3 the age of the Earth. “With five well-studied events at hand,” the authors write, “for the first time there are enough data in this interesting redshift range to make a direct and statistically significant empirical study.”

The findings: Gamma ray bursts occur only in metal-poor environments. Large spiral galaxies like the Milky Way have been too metal-rich to host a GRB for several billion years. That seems to rule out the scenario of a nearby GRB (a few kiloparsecs away) causing mass extinctions on Earth. From the paper:

Our results make this scenario most unlikely — by the time the Earth formed, the Milky Way disk was already too metal-rich to host a long GRB. SN 1998bw/GRB 980425, the only local event to happen in a fairly metal-enriched galaxy, was also by far the weakest localized GRB ever, with at least 10,000 times lower energy than a typical z ~ 1 GRB [the value z is the redshift parameter]. As such, it would not cause mass extinction at several kpc from Earth. The same can be said about short GRBs, which are not only less frequent than long GRBs…but also less energetic and less beamed… Short GRBs are also not concentrated to star-forming regions, thus on average they are much further away from any life-hosting planets…

Something else to consider is that planet-forming stars tend to be even more metal rich than our Sun, making long GRBs an unlikely factor in life extinction events anywhere near us. “So to finish with a bit of good news,” write the authors, “we can probably cross GRBs off the rather long list of things that could cause humankind to ‘join the dinosaurs’ on the extinct species list.”

Sharp-eyed readers will remember Kris Stanek’s earlier work on the Hubble Constant, suggesting that it be revised based on his team’s studies of the Triangulum Galaxy (M33). The current paper is Stanek et al., “Protecting Life in the Milky Way: Metals Keep the GRBs Away,” slated for publication in Acta Astronautica and available in preprint form online. It adds to the growing consensus about GRBs as a threat, though as we’ve recently seen, just what kind of supernova causes long-duration GRBs is still controversial.

Schedule Update

Merry Christmas! And for those of you who celebrate other holidays — or none at all — best wishes for the season. Centauri Dreams will not publish on December 24 or 25. The normal publication schedule resumes on the afternoon of the 26th.

A Gamma Ray Puzzle

When you’re calculating the odds on life in any region of the galaxy, the rate of supernova explosions comes into play. As we saw yesterday, one factor Nikos Prantzos examined in his recent work on the galactic habitable zone was the effect that hard radiation could have on exposed land life. But what about gamma ray bursts (GRBs)? They’re more powerful and, although rarer than supernovae, can create beamed energy that makes them lethal from larger distances.

One theory is that because gamma ray bursts are associated with regions of low metallicity outside our galaxy, their frequency in the Milky Way is now close to zero. But a reminder of how little we actually know comes in the December 21 Nature, where four papers discuss GRB activity, and in particular a burst picked up by NASA’s Swift satellite last June 14. It’s a cosmic oddity, a kind of hybrid that probably marks the birth of a black hole. But, as Derek Fox (Penn State) says, “This burst — unlike all other long gamma-ray bursts we have seen at close distance — was not accompanied by a supernova, for reasons we do not yet fully understand.”

The current dichotomy for gamma ray bursts runs something like this: long GRBs last more than two seconds and appear to mark the birth of a black hole resulting from a supernova explosion. Short GRBs may be as brief as several milliseconds, and apparently mark the merger of two neutron stars, or a neutron star and a black hole.

The trick with the recent GRB 060614 burst is that it was a long one — 102 seconds — from a place (some 1.6 billion light years away in the constellation Indus) where star formation rates are low and few stars are massive enough to produce supernovae. Moreover, no trace of a supernova can be found there. “This burst was close enough to detect a supernova if it existed,” said Avishay Gal-Yam (Caltech), lead author on one of the Nature papers, “but even Hubble didn’t see anything.”

Equally interesting is GRB 060505, a burst detected last May. That one, originating in a galaxy 1 billion light years away in the direction of the constellation Piscis Austrinus, also left no supernova remnant.

Maybe the merger model needs an overhaul. Or if supernovae were involved in both events, then they may have formed black holes that allowed absolutely no matter to escape, the usual supernova ejecta being completely consumed. In any case, some new process seems to be at work in the two bursts, one that calls into question our understanding of GRBs and therefore our ability to predict their future frequency with accuracy. And it underscores the difficulty in estimating the long-term effects of GRBs on possible extraterrestrial life.

The paper referenced above is Gal-Yam et al., “A novel explosive process is required for the bold gamma-ray burst GRB 060614,” Nature 444 (21 December 2006), pp. 1053-1055 (abstract here). Three other short papers discuss the recent GRB findings in the same issue.

Life Throughout the Galaxy?

Is there a galactic habitable zone, a region within the Milky Way where conditions for life are optimum? If so, we want to know its parameters, as they would help us define the search area for living worlds. The concept has kicked around for a while, and now surfaces again in an interesting paper by Nikos Prantzos (Institut d’Astrophysique de Paris). Prantzos ponders the main variables and, while concluding that the galactic habitable zone is far from well understood, believes it conceivable that the entire galactic disk may, at this stage of its evolution, be suitable for life.

That conclusion goes further than Charles Lineweaver and team’s work in 2004, the latter having found that the zone for complex life exists in a ring a few kiloparsecs wide surrounding the galactic center and gradually spreading outward as the Galaxy evolves (our earlier story on that work and habitable zones in general is here). Like Lineweaver, Prantzos looks at planet formation in terms of stellar metallicity (the amount of elements heavier than hydrogen and helium in the body of the star), using later data based on simulations of planet formation.

Artist's conception of the Milky Way

Image: An artist’s conception of the Milky Way. Can the entire galactic disk now be considered a viable habitable zone? Credit: NASA, ESA and The Hubble Heritage Team (AURA/STScI).

So what do we really know about metallicity? Stars hosting planets show high metallicity compared to stars that have no planets, a trend that points to metallicity as a major factor in gas giant formation. But note: the effect of metallicity on Earth-like worlds is unknown, and some theories even suggest that low metallicity may stimulate terrestrial planet formation. If that work (still unpublished) bears out, then Earth-like planets may be common in low metallicity environments like the outer edges of the galaxy.

Another key factor is the risk of life being exterminated by nearby supernovae events. A hostile cosmic environment, in other words, could sharply limit the number of life-bearing worlds. But Prantzos argues that the supernova threat is hard to characterize. If all land animals on a planet die because of nearby supernova radiation, marine life will most likely survive. Here the author comments on the survivability question:

In the case of the Earth, it took just a few hundred million years for marine life to spread on the land and evolve to dinosaurs and ultimately, to humans; this is less than 4% of the lifetime of a G-type star. Even if land life on a planet is destroyed from a nearby SN explosion, it may well reappear again after a few 108 years or so. Life displays unexpected robustness and a cosmic catastrophe might even accelerate evolution towards life forms that are presently unknown. Only an extremely high frequency of such catastrophic events (say, more than one every few 107 yr) could, perhaps, ensure permanent disappearance of complex life from the surface of a planet.

Then factor in galactic evolution, which suggests that the rate of supernovae, although always higher in the inner disk, declines over time, and survivability in the inner disk regions goes up as the galaxy ages. The ‘ring’ of survivability is narrow early on but progressively migrates outwards, extending perhaps to the outer rim of the galaxy today. There follows this interesting consequence:

Thus, despite the high risk from SN early on in the inner disk, that place becomes later relatively ‘hospitable.’ Because of the large density of stars in the inner disk, it is more interesting to seek complex life there than in the outer disk; the solar neighborhood (at 8 kpc from the center) is not privileged in that respect.

Prantzos is no dogmatist. A clear-eyed theorist, he takes pains to note that these findings depend heavily on our assumptions about metallicity and planet formation (and the question of migrating ‘hot Jupiters’ weighs in significantly as well). But if most of the galaxy really is suitable for life, then the concept of a galactic habitable zone loses punch. We may find that our search for extraterrestrial life can roam broadly through the cosmos.

The paper is Prantzos, Nikos, “On the ‘Galactic Habitable Zone,'” slated for publication in Space Science Reviews and available as a preprint online. The Lineweaver paper is “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way,” Science Vol. 303, No. 5654 (2 January 2004), pp. 59-62, with abstract here.

Fermi Meets Elsie: SETI’s New Conundrum

“Simple and cheap, like onion dip.” That’s how Seth Shostak (SETI Institute) refers to our early optical search systems, which have involved limited equipment in the hunt for extraterrestrial intelligence, at least when compared to the much more demanding resources deployed by the radio search. Cheap is good, but not when you can only check one part of the sky at a time. All this gets Shostak pondering in a recent article about the parameters of a laser signal from an extraterrestrial civilization.

For if we might miss a faint signal, what about a really big one? Suppose an intelligent species somewhere out there is deliberately trying to contact our planet. Wouldn’t it make sense, Shostak muses, to create a huge optical impression, a signal that would catch our attention so obviously that we could then focus in to detect whatever message might be streaming from that same location? Bright objects in the sky do appear and are usually recorded, as witness historical records of supernovae.

And so it may be telling us something that we have no records of recurring bright objects. Sure, it would take huge resources to make a signal from such a civilization bright enough for the average person to see it without any equipment (Shostak estimates 5 X 1025 watts to push such a signal from 1000 light years away). That’s well beyond our resources, but not those of a Kardashev Type II civilization (one capable of using the entire power output of its Sun), which could imply there is nothing more advanced than a Type I civilization near us.

But whatever its Kardashev type, an advanced civilization may have no interest in beaming a signal to us in the first place. Or perhaps we remain simply undiscovered in a galactic backwater. Whatever the case, ‘naked eye SETI’ adds another twist to the ‘where are they’ question that Fermi posed, and at least seems to be saying that if a Type II culture wanted to reach us, it could have made its presence so blindingly obvious that we would be sure not to miss it. “…it strikes me as paradoxical,” says Shostak, “given the vastness of the cosmos, that such a simple signal has not been recognized, a signal that even a cow could see.” Welcome to the ‘Cow Paradox.’

Centauri Dreams‘ take: Long-time readers know I think there are few technological civilizations in our galaxy to be detected. When asked, I always settle on a number like 5-10 instead of Sagan’s 1 million. That’s the thought of a writer with no scientific qualifications other than a keen interest in these topics. But we’re all just guessing at this point, and this writer is not at all surprised our SETI efforts have so far come up short.